Add support for NoTagAccess memory attribute

[PATCH 3/4] arm64: mte: update code comments

Posted by Aneesh Kumar K.V (Arm) 3 weeks, 6 days ago

commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag
initialisation") updated the locking such the kernel now allows
VM_SHARED mapping with MTE. Update the code comment to reflect this.

Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 arch/arm64/kvm/mmu.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index a509b63bd4dd..b5824e93cee0 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1390,11 +1390,8 @@ static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
  * able to see the page's tags and therefore they must be initialised first. If
  * PG_mte_tagged is set, tags have already been initialised.
  *
- * The race in the test/set of the PG_mte_tagged flag is handled by:
- * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
- *   racing to santise the same page
- * - mmap_lock protects between a VM faulting a page in and the VMM performing
- *   an mprotect() to add VM_MTE
+ * The race in the test/set of the PG_mte_tagged flag is handled by
+ * using PG_mte_lock and PG_mte_tagged together.
  */
 static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
 			      unsigned long size)
@@ -1646,7 +1643,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	}
 
 	if (!fault_is_perm && !device && kvm_has_mte(kvm)) {
-		/* Check the VMM hasn't introduced a new disallowed VMA */
+		/*
+		 *  not a permission fault implies a translation fault which
+		 *  means mapping the page for the first time
+		 */
 		if (mte_allowed) {
 			sanitise_mte_tags(kvm, pfn, vma_pagesize);
 		} else {
-- 
2.43.0

Re: [PATCH 3/4] arm64: mte: update code comments

Posted by Marc Zyngier 3 weeks, 6 days ago

On Mon, 28 Oct 2024 09:40:13 +0000,
"Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org> wrote:
> 
> commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag
> initialisation") updated the locking such the kernel now allows
> VM_SHARED mapping with MTE. Update the code comment to reflect this.
> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>  arch/arm64/kvm/mmu.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)

This is a KVM patch. Please make sure you write the subject
accordingly, matching the existing conventions (in this case, this
should read something like: "KVM: arm64: MTE: Update...").

> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index a509b63bd4dd..b5824e93cee0 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1390,11 +1390,8 @@ static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
>   * able to see the page's tags and therefore they must be initialised first. If
>   * PG_mte_tagged is set, tags have already been initialised.
>   *
> - * The race in the test/set of the PG_mte_tagged flag is handled by:
> - * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
> - *   racing to santise the same page
> - * - mmap_lock protects between a VM faulting a page in and the VMM performing
> - *   an mprotect() to add VM_MTE
> + * The race in the test/set of the PG_mte_tagged flag is handled by
> + * using PG_mte_lock and PG_mte_tagged together.

How? This comment is pretty content-free. TO be useful, you should
elaborate on *how* these two are used together.

>   */
>  static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>  			      unsigned long size)
> @@ -1646,7 +1643,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	}
>  
>  	if (!fault_is_perm && !device && kvm_has_mte(kvm)) {
> -		/* Check the VMM hasn't introduced a new disallowed VMA */
> +		/*
> +		 *  not a permission fault implies a translation fault which
> +		 *  means mapping the page for the first time

How about an Access fault due to page ageing?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

Re: [PATCH 3/4] arm64: mte: update code comments

Posted by Aneesh Kumar K.V 3 weeks, 6 days ago

Hi Marc,

Thanks for reviewing the changes.

Marc Zyngier <maz@kernel.org> writes:

> On Mon, 28 Oct 2024 09:40:13 +0000,
> "Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org> wrote:
>> 
>> commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag
>> initialisation") updated the locking such the kernel now allows
>> VM_SHARED mapping with MTE. Update the code comment to reflect this.
>> 
>> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>> ---
>>  arch/arm64/kvm/mmu.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> This is a KVM patch. Please make sure you write the subject
> accordingly, matching the existing conventions (in this case, this
> should read something like: "KVM: arm64: MTE: Update...").
>

Will update

>
>> 
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index a509b63bd4dd..b5824e93cee0 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1390,11 +1390,8 @@ static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
>>   * able to see the page's tags and therefore they must be initialised first. If
>>   * PG_mte_tagged is set, tags have already been initialised.
>>   *
>> - * The race in the test/set of the PG_mte_tagged flag is handled by:
>> - * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
>> - *   racing to santise the same page
>> - * - mmap_lock protects between a VM faulting a page in and the VMM performing
>> - *   an mprotect() to add VM_MTE
>> + * The race in the test/set of the PG_mte_tagged flag is handled by
>> + * using PG_mte_lock and PG_mte_tagged together.
>
> How? This comment is pretty content-free. TO be useful, you should
> elaborate on *how* these two are used together.
>

I will add more details described in commit d77e59a8fccde7fb5dd8c57594ed147b4291c970
Should i quote the commit there in the comment? 

>
>>   */
>>  static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>>  			      unsigned long size)
>> @@ -1646,7 +1643,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>  	}
>>  
>>  	if (!fault_is_perm && !device && kvm_has_mte(kvm)) {
>> -		/* Check the VMM hasn't introduced a new disallowed VMA */
>> +		/*
>> +		 *  not a permission fault implies a translation fault which
>> +		 *  means mapping the page for the first time
>
> How about an Access fault due to page ageing?
>

IIUC access fault is already handled by the caller kvm_handle_guest_abort?
I can add that as part of the updated comments?

-aneesh

Re: [PATCH 3/4] arm64: mte: update code comments

Posted by Marc Zyngier 3 weeks, 6 days ago

On Mon, 28 Oct 2024 12:47:30 +0000,
Aneesh Kumar K.V <aneesh.kumar@kernel.org> wrote:
> 
> >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> >> index a509b63bd4dd..b5824e93cee0 100644
> >> --- a/arch/arm64/kvm/mmu.c
> >> +++ b/arch/arm64/kvm/mmu.c
> >> @@ -1390,11 +1390,8 @@ static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
> >>   * able to see the page's tags and therefore they must be initialised first. If
> >>   * PG_mte_tagged is set, tags have already been initialised.
> >>   *
> >> - * The race in the test/set of the PG_mte_tagged flag is handled by:
> >> - * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
> >> - *   racing to santise the same page
> >> - * - mmap_lock protects between a VM faulting a page in and the VMM performing
> >> - *   an mprotect() to add VM_MTE
> >> + * The race in the test/set of the PG_mte_tagged flag is handled by
> >> + * using PG_mte_lock and PG_mte_tagged together.
> >
> > How? This comment is pretty content-free. TO be useful, you should
> > elaborate on *how* these two are used together.
> >
> 
> I will add more details described in commit d77e59a8fccde7fb5dd8c57594ed147b4291c970
> Should i quote the commit there in the comment?

The commit is not relevant. What is important is an indication of how
the race is resolved if that's important. A reference to
try_page_mte_tagging() would probably be the right thing to do.

> 
> >
> >>   */
> >>  static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
> >>  			      unsigned long size)
> >> @@ -1646,7 +1643,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >>  	}
> >>  
> >>  	if (!fault_is_perm && !device && kvm_has_mte(kvm)) {
> >> -		/* Check the VMM hasn't introduced a new disallowed VMA */
> >> +		/*
> >> +		 *  not a permission fault implies a translation fault which
> >> +		 *  means mapping the page for the first time
> >
> > How about an Access fault due to page ageing?
> >
> 
> IIUC access fault is already handled by the caller kvm_handle_guest_abort?
> I can add that as part of the updated comments?

Maybe. The thing is, you are removing a pretty essential comment for
no good reason, and now there is no rational left behind the -EFAULT
that is returned.

	M.

-- 
Without deviation from the norm, progress is not possible.